Lignocellulosic biomass is viewed as an abundant renewable resource for chemicals due to the presence of sugars in the cell walls of plants. More than 50% of the organic carbon on the earth's surface is contained in plants. This lignocellulosic biomass is comprised of hemicelluloses, cellulose and smaller portions of lignin and protein. These structural components are comprised primarily of pentose and hexose sugars monomers. Cellulose is a polymer comprised mostly of condensation polymerized glucose and hemicellulose is a precursor to pentose sugars, mostly xylose. These sugars can be converted into valuable components, provided they can be liberated from the cell walls and polymers that contain them. However, plant cell walls have evolved considerable resistance to microbial, mechanical or chemical breakdown to yield component sugars. In order to overcome recalcitrance ground biomass is altered by a chemical process known as pretreatment. The aim of the pretreatment is to hydrolyze the hemicellulose, break down the protective lignin structure and disrupt the crystalline structure of cellulose. All of these steps enhance enzymatic accessibility to the cellulose during the subsequent hydrolysis (saccharification) step.
The original approaches dating back to the early 19th century involve complete chemical hydrolysis using concentrated mineral acids such as hydrochloric acid, nitric, or sulfuric acid. Numerous improvements to these processes have been made earning higher sugar yields from the biomass feedstock. These higher acid concentration approaches provide higher yields of sugars, but due to economic and environmental reasons the acids must be recovered. The primary obstacle to practicing this form of saccharification has been the challenges associated with recovery of the acid [M. Galbe and G. Zacchi, Appl. Microbiol. Biotechnol. Vol. 59, pp. 618-628 (2002)]. Recent efforts toward separating sulfuric acid and sugars using ion resin separation or hydrochloric acid and sugars via an amine extraction process and subsequent thermal regeneration of the acid have been described in U.S. Pat. No. 5,820,687. However, both of these approaches are cumbersome and expensive in practice.
Dilute acid processes have also been attempted to perform chemical saccharification and one such example is the Scholler-Tornesch Process. However usage of dilute acid requires higher temperatures and this usually results in low yields of the desired sugars due to thermal degradation of the monsaccharides. Numerous approaches of this type have been made in the past and all have failed to meet economic hurdles. [See, for example, Lim Koon Ong, “Conversion of Lignocellulosic Biomass to Fuel Ethanol—A Brief Review,” The Planter, Vol. 80, No. 941, August 2004, and, “Cell Wall Saccharification,” Ralf Moller, in Outputs from the EPOBIO Project, 2006; Published by CPL Press, Tall Gables, The Sydings, Speen, Newbury, Berks RG14 1RZ, UK].
The saccharification of the cellulose enzymatically holds promise of greater yields of sugars under milder conditions and is therefore considered by many to be more economically attractive. The recalcitrance of the raw biomass to enzymatic hydrolysis necessitates a pretreatment to enhance the susceptibility of the cellulose to hydrolytic enzymes. A number of pretreatment methods, such as described by Mosier, et al. [Bioresource Technology, Vol. 96, pp. 673-686 (2005)], have been developed to alter the structural and chemical composition of biomass to improve enzymatic conversion. Such methods include treatment with a dilute acid steam explosion, as described in U.S. Pat. No. 4,461,648, hydrothermal pretreatment without the addition of chemicals as described in WO 2007/009463 A2, ammonia freeze explosion process as described by Holtzapple, M. T., et al. [Applied Biochemistry and Biotechnology, 28/29, pp. 59-74], and an organosolve extraction process described in U.S. Pat. No. 4,409,032. Despite these approaches, such pretreatment has been cited as the most expensive process in biomass-to-fuels conversion [Ind. Eng. Chem. Res., Vol. 48(8), 3713-3729 (2009)].
One pretreatment that has been extensively explored is a high temperature, dilute-sulfuric acid (H2SO4) process, which effectively hydrolyzes the hemicellulosic portion of the biomass to soluble sugars and exposes the cellulose so that enzymatic Saccharification is successful. The parameters which can be employed to control the conditions of the pretreatment are time, temperature, and acid loading. These are often combined in a mathematical equation termed the combined severity factor. In general, the higher the acid loading employed, the lower the temperature that can be employed; this comes at a cost of acid and its need to recycle the acid. Conversely, the lower the temperature, the longer the pretreatment process takes; this comes at the cost of productivity. However the use of the higher concentrations of acid required to lower the pretreatment temperatures below that where furfural formation becomes facile [B. P. Lavarack, et al., Biomass and Bioenergy, Vol. 23, pp. 367-380 (2002)] once again requires the recovery of the strong acid. If dilute acid streams and higher temperatures are employed the pretreatment reaction the acid passing downstream to the enzymatic hydrolysis and subsequent fermentation steps must be neutralized resulting in inorganic salts which complicates downstream processing and requires more expensive waste water treatment systems. This also results in increased chemical costs for acid and base consumption.
More recently, in US20120122152, α-hydroxysulfonic acids have been shown to be effective in the pretreatment and hydrolysis of biomass with the additional benefit of being recoverable and recyclable through reversal to the acids primary components (aldehyde, SO2 and water). This pretreatment process has been shown to provide numerous benefits compared to dilute mineral acid pretreatment. However, at the low temperature, the formation of furfural is low.
A method for preparing furfural may use a batch process based on a Quaker Oats technology developed in 1920 using sulfuric acid. The batch process is known to be significantly inefficient. That is, the theoretical furfural yield is about 30 to 40%, the residence time in the reactor is significant long as 4.5 to 5.5 hours, water of 50 MT per 1 MT of furfural is consumed, and a significant amount of harmful substance is included in effluents. In addition, costs consumed for working are considerably increased
Further, whether batch or continuous, when using such acid catalyst, the process corrosion and the acid wastes are generated, such that it is difficult to separate, recover, and recycle a non-reactive raw material and the acid catalyst. Further, the economic efficiency of the process may be very vulnerable according to the increase in investment costs of process facility and low product yield and environmental toxicity, recovery, and recycle may be complicated even in the process of using an organic solvent